A woman who represents the serial killer in his high-security hospital was arrested after claiming in a television documentary that he gave her a sealed letter explaining where Keith Bennett’s body was buried almost 50 years ago.

Detectives who have searched Jackie Powell’s house as well as Brady’s room have found no trace of the document.

The mental health advocate claims she returned it to him instead of giving it to the dead boy’s mother as instructed.

Suspicions are growing that it might have been another attempt by the 74-year-old murderer to manipulate the authorities and torment the grieving relatives of his victims. However, police are continuing to examine seized documents.

“What we are looking at is the possibility, and at this stage it is only a possibility, that he has written a letter to Keith's mum, Winnie Johnson, which was not to be opened until after his death.

“We do not know if this is true or simply a ruse but we clearly have a duty to investigate such information on behalf of Keith's family.”

Mrs Powell, who was arrested on Thursday before being bailed, herself admitted in the documentary, to be broadcast next week: “Well to be perfectly honest with you there might be nothing in the letter – many games have been played before. That is the mind of a psychopath.”

But she also said: “Every human being, whoever they are, should be treated with some amount of dignity and respect.”

John Ainley, the solicitor representing Keith’s mother, said any information about the location of his body should be handed over but added: “The family are very sceptical as to whether there is a letter and as to the contents of it.”

He said Mrs Johnson, who has cancer, has always believed that Brady knows exactly where the body lies and added: “It beggars belief that he would not want to pass that information on.”

David Kirwan, another solicitor who formerly represented the family, agreed: “I believe Ian Brady knows the exact whereabouts of Keith Bennett's grave and is capable of directing police to it.

“However, it is important to remember who we are dealing with and how he seems to enjoy bizarre mind games and manipulation.”

Brady and his girlfriend Myra Hindley were jailed for life over three murders in 1966 and the bodies of some of their child victims were found buried on Saddleworth Moor near Oldham at the time.

But it was not until the mid-1980s that he confessed to two more killings and although the remains of Pauline Reade were discovered on the moors, those of Keith Bennett, who was 12 when he went missing in 1964, were not found.

Brady also wrote to the BBC claiming to have committed five other murders, and has also sent a letter to Keith’s mother, Winnie Johnson.

He has spent years on hunger strike as part of what a judge called his “obsessive need to exercise control”, and he was due to appear in public last month in a bid to be sent to a Scottish prison and be allowed to die.

In the new documentary, it is claimed that Brady once used method acting techniques to trick prison authorities into moving him into a hospital by “mimicking other patients”.

While being interviewed for the programme Mrs Powell, who has acted on Brady’s behalf since 1999 but is neither a solicitor nor an employee of Ashworth hospital in Merseyside where he is detained, claimed he had given her a sealed envelope to pass to Keith’s mother in the event of his death.

“Within that is the means of her possibly being able to rest,” said Mrs Powell.

The production team told Greater Manchester Police on July 30th and on Thursday the 49-year-old was detained on suspicion of preventing the burial of a body without lawful excuse.

However it is now believed that she claims to have returned the letter to Brady before her arrest.

Detectives have examined a large number of documents seized from her home in South Wales, and have also searched Brady’s hospital quarters, but found no sign of any such letter.

Mrs Powell, who has two teenage children, told Sky News on Friday that her words had been “misrepresented” and “blown out of all proportion”.

She is on police bail while the documentary, called Ian Brady: Endgames Of A Psychopath, will be broadcast on Channel 4 at 9pm on Monday.